'Emerge stronger'

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Journalists and TV crews gathered outside the home of Mr Sotloff's parents in Florida ahead of the news conference

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Islamic State militants have declared a Caliphate in Iraq and Syria, led by Abu Bakr al-Baghdadi

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Mr Biden said that the US would not stop until the militants are brought to justice

He said that Mr Sotloff "was no war junkie and did not want to be a modern-day Lawrence of Arabia".

"From the Libyan doctor in Misrata who struggled to provide psychological services to children ravaged by war, to the Syrian plumber who risked his life by crossing regime lines to purchase medicine, their story was Steve's story. He ultimately sacrificed his life to bring their story to the world," Mr Barfi said.

"Today we grieve but we will emerge stronger. We will not allow our enemies to hold us hostage with the sole weapon they possess: Fear."

Mr Barfi said that Steven Sotloff was "torn between two poles" - his love of home life and his passion for the Arab world.

He said that the Sotloff family's prayers went out to the family of Jim Foley - another American journalist who was similarly killed last month.

"Like Steve he suffered but his jailors never broke him. He was an inspiration to others held in prison."

'Degrade and destroy'

The tribute to Mr Sotloff came after President Barack Obama vowed that the US would not be intimidated following the release of a video tape of his murder by Islamic State (IS) militants.

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Media captionSteven Sotloff had a passion for the Middle East and a taste for adventure, as the BBC's Frank Gardner reports

Mr Obama said the US would build a coalition to "degrade and destroy" IS.

Vice-President Biden said that the US would not stop until the militants are brought to justice.

He said the American people "are so much stronger, so much more resolved" than any enemy can understand.

Next to a masked figure, Mr Sotloff reads out a text addressed to President Obama saying: "You've spent billions of US taxpayers' dollars and we have lost thousands of our troops in our previous fighting against the Islamic State, so where is the people's interest in reigniting this war?"

The militant spoke with a British accent similar to that of the man who appeared to carry out the beheading of James Foley.

He also threatens to kill the British hostage shown in the footage. The family of the British hostage have asked the media not to release his name.

IS has seized large swathes of territory in Iraq and Syria in recent months, declaring a new caliphate, or Islamic state, under leader Abu Bakr al-Baghdadi.

The US has launched more than 120 air strikes in the past month to try to help Kurdish forces curb the IS advance.